3D laser scanning techniques are commonly used for terrestrial-based 3D mapping and metrology applications. Multiple data sets covering different, generally overlapping areas are collected and then registered and aggregated to form larger data sets. The techniques for collecting and registering laser data, including surface matching of overlapping data sets, take advantage of the high resolution of the data and the availability of distinctive features within the data sets. Co-registration of GPS and LIDAR generated data may improve the accuracy of data sets. Methods for registration of LIDAR data with optical imagery to improve 3D modeling simulations are also known.
Underwater terrain is typically mapped using acoustic bathymetry systems. Multibeam echosounders, which generally incorporate many narrow adjacent beams arranged in a fan-like swath, can provide high angular resolution and accuracy. The echosounders are generally mounted on vessels that steer overlapping swaths over the target terrain to collect mapping data. The beams may update frequently, allowing reasonably fast vessel speed while maintaining full coverage of the seafloor. Attitude sensors mounted on the vessel may provide data for correction of the boat's roll, pitch and yaw on the ocean surface, and a gyrocompass may be provided for accurate heading information. The Global Positioning System (or other Global Navigation Satellite System (GNSS)) may be used to position the soundings with respect to the surface of the earth. Sound speed profiles (speed of sound in water as a function of depth) of the water column may be collected to correct for refraction or “ray-bending” of the sound waves as a result of non-uniform water column characteristics such as temperature, conductivity, and pressure. A computer system may process all or a portion of this data, correcting for the various factors, as well as for the angle of each individual beam. The resulting sounding measurements are then processed manually, semi-automatically or automatically (in limited circumstances) to produce a map of the underwater terrain.
While the acoustic bathymetry systems described above provide adequate resolution for many purposes, higher resolution underwater mapping and metrology has been difficult to achieve. Optical video cameras are well known and have been in use for many years. Underwater optical video cameras generally use analog or digital video transmission techniques and can provide satisfactory resolution and viewing range in illuminated, clear underwater conditions. Underwater conditions are often not conducive to optical cameras, however, because illumination is poor or because the water lacks clarity. Other types of optoelectronic sensors have also been developed, including various types of spectroscopic systems. Sensors that rely on optics, however, such as spectroscopic systems, have limited viewing range in dark, murky or turbid underwater conditions. For this reason, they have limited use in underwater environments and the information they provide can be quite unreliable.
High resolution imaging sonar (i.e., acoustic) systems have been available and are used in many different underwater applications. Sonar imaging systems provide satisfactory resolution and viewing range in many different water conditions, including murky and turbid underwater conditions. Imaging sonar systems designed for visualization and/or object detection generally scan a fan-shaped beam in a given area by rotating the system or translating it along a line, generating data points and forming images in the direction of the beam rotation or translation. In general, the angle through which the beam is moved is relatively small, the fan-shaped beam has a narrow angle, and the transmitted pulse is short.
Multi-beam sonar systems, including frequency-steerable multi-beam sonar systems, are well known and may be used for many types of 2D and 3D imaging applications, as well as bathymetry, object detection, surveillance and other types of applications. Exemplary frequency-steerable multi-beam acoustic systems, and other types of imaging systems, are disclosed in U.S. Pat. Nos. 7,606,114, 7,542,376 and 7,889,600, which are incorporated herein by reference in their entireties. These systems are capable of providing high resolution underwater imagery.
Underwater installations of various types, such as pipelines, cabling, tunnels, support structures, rigs and the like may require precise, high resolution measurement of underwater components and structures for installation, monitoring and repair purposes. Spoolpiece metrology, which is the measurement of the relative separation and angle between flanges where underwater pipe sections are connected, or where pipe sections are connected to fixed structures, is one exemplary underwater metrology application that requires high resolution underwater data collection. This metrology application requires highly accurate and precise data, to ensure that pipe and flange connections match and to accurately determine dimensional flange and pipe fabrication requirements.
One standard method of spoolpiece metrology utilizes Long Baseline (LBL) transponders placed in locations near the pipeline or underwater structure of interest and measures ranges between the transponders very accurately. An array of 6-10 transponders is typically deployed on the seabed, on the pipeline, structure and on the flanges. All the inter-transponder ranges are measured and used to compute the relative position and bearing between the flanges. This process, including deployment, range measurement and recovery, generally requires significant vessel time (on the order of 12-18 hours), with vessel costs often ranging between £60,000 and £110,000 per day. This system, while accurate, is a very high cost and time intensive system.
Using a sonar system to collect precise dimensional and relational data could dramatically reduce the amount of vessel time required and, thus, the cost in various underwater metrology applications. Use of a high resolution sonar system may provide more complete, higher resolution data and imaging as well. This type of system would be useful for many types of underwater metrology, object and structure detection, survey and evaluation applications. Techniques for registration of multiple data sets are required for implementation of such high resolution imaging sonar systems in underwater metrology applications.
Registration of underwater acoustic data with terrestrial data collected using laser-based systems is also challenging and, if done accurately, would provide comprehensive survey and mapping information. Because laser-based data is generally collected above the water line, and acoustic data is generally collected below the water line, and because there are few fixed, distinctive features at or near the water-air interface, registration of above the waterline and below the waterline data sets using common data features isn't feasible. Tidal and water level fluctuations, and wave action, eliminate the use of simple waterline-based registration systems for registering above and below the water line data sets, even when adjusted to account for tidal fluctuations. Registration systems that provide accurate registration of above waterline data sets with underwater data sets would also be useful for many types of survey, mapping and metrology applications.